1. Field of the Invention
The present invention relates to a sheet feeder for feeding sheets toward an image transfer station one by one. More particularly, the present invention relates to a sheet feeding device capable of again feeding, when failed to feed a sheet, the same sheet by retry control and a printer including the same.
2. Description of the Background Art
A sheet feeding device included in a stencil printer or similar printer is constructed to feed sheets stacked on a sheet tray with a feed roller or pickup roller one by one toward a registration roller pair, the top sheet being first. A leading edge sensor responsive to the leading edge of the sheet is positioned upstream of the registration roller pair in the direction of sheet feed. When the leading edge sensor senses the leading edge of the sheet, the feed roller further conveys the sheet by a preselected distance until the leading edge of the sheet abuts against the nip of the registration roller pair.
The preselected distance mentioned above is longer than a distance between the leading edge sensor and the nip of the registration roller pair, so that the sheet is caused to form a loop due to excessive feed and has its skew corrected thereby. The registration roller pair selectively opens or closes in synchronism with the rotation of a print drum or similar image carrier, conveying the sheet to an image transfer station such that the preselected position of the sheet meets the leading edge of an image.
When the leading edge sensor does not sense the leading edge of the sheet within a preselected period of time, a controller included in the printer determines that the sheet feeding device has failed to feed to sheet (jam), and urges the operator of the printer to remove the sheet.
However, it is time- and labor-consuming for the operator to stop the operation of the printer and then remove the jamming sheet. Particularly, in a stencil printer that usually outputs a number of prints, the operator often leaves the printer over a long period of time until the end of printing. In this respect, a feed failure occurred in the absence of the operator results in a substantial time loss. In light of this, it is a common practice to execute so-called retry control, or refeed control, for again feeding a sheet not fed due to a failure to thereby prevent the operation of the printer from being interrupted as far as possible.
Japanese Patent Laid-Open Publication No. 7-277553, for example, discloses a document conveying device with a retry control capability and configured to again convey, when a document is not sensed in a preselected period of time, the document at half a speed. By reducing the conveying speed, the document conveying device increases friction to act between the document and a conveyor roller and therefore a conveying ability.
Japanese Patent Laid-Open Publication No. 8-169632 teaches a sheet feeding device constructed to increase, in the event of retry control, the drive speed of a drive motor assigned to a feed roller, thereby again feeding a sheet not fed due to a failure without lowering the overall printing speed. Further, Japanese Patent Laid-Open Publication No. 2000-132002 proposes an image forming apparatus constructed such that when a sheet is not sensed within a preselected period of time, the sheet not fed due to a failure is again fed on the basis of information output from scanning optics at the time when the optics completed scanning one time performs the next scanning.
However, the conventional sheet feeding schemes have the following problem left unsolved. A position at which a sheet not fed due to a failure is stopped is not constant. It is therefore likely that the refeed timing of such a sheet is not matched to the opening/closing timing of a registration roller pair, which is synchronous to the rotation timing of the image carrier. Any error in refeed timing directly translates into the shift of an image on the sheet.
Technologies relating to the present invention are also disclosed in, e.g., U.S. Pat. No. 6,298,778.
It is an object of the present invention to provide a sheet feeding device capable of accurately effecting the refeed of a sheet not fed due to a failure and a printer including the same.
A sheet feeding device of the present invention capable of again feeding, when failed to feed a sheet, the sheet by retry control includes a sheet tray loaded with a stack of sheets and a feed roller for sequentially feeding the sheets from the sheet tray one by one. A leading edge sensor is positioned downstream of the feed roller in the direction of sheet feed for sensing the leading edge of the sheet fed from the sheet tray. A registration roller pair is positioned downstream of the leading edge sensor in the direction of sheet feed for conveying the sheet toward an image transfer station at a preselected timing. When the retry control is to be executed, control means varies the content of control in accordance with information output from the leading edge sensor.
A printer including the sheet feeding described above device is also disclosed.